The need for sustainable, carbon-neutral technologies to meet the world's growing demand for energy has become a widely acknowledged imperative. However, despite this realization, renewable energy sources such as wind and solar power remain under-exploited as large-scale replacements for fossil fuels. This is largely due to the intermittency of supply: a reliable way of storing the energy generated by renewable sources must be found in order to compensate for periods during which the wind does not blow or the sun does not shine. An especially attractive solution to this problem is to store sustainably-generated energy in the form of chemical bonds by the electrochemical splitting of water to produce oxygen (O2) and hydrogen (H2).
Oxygen-evolving reaction (OER):2H2O→O2+4H++4e−Eoxidation=−1.23 V  (Eq. 1)
Hydrogen-evolving reaction (HER):4H++4e−→2H2Ereduction=0.0 V  (Eq. 2)
Equations 1 and 2 give the half-reactions for water-splitting at room temperature at pH 0, showing that the theoretical minimum voltage which must be supplied to drive the OER and the HER simultaneously is 1.23 V. In practice, however, additional energy (hereafter termed “overpotential”) must be supplied to surmount various systemic and kinetic barriers, which means that voltages well in excess of 1.6 V are typically needed in order to provide simultaneous OER and HER.
There is a desire to develop technologies that allow the production of hydrogen and oxygen at lower effective voltages.
A particular focus of the research into hydrogen and oxygen production is the utilisation of light as an energy source for the electrochemical reaction. In some circumstances this may simply be the use of photovoltaic cells to provide the bias in a hydrogen- and oxygen-producing electrochemical cell. However, there is now a real interest in the use of photo electrochemical cells for hydrogen and oxygen production. Thus a photo responsive material is present within the cell itself, and its photochemical response drives the electrochemistry within the cell. The photo-responsive material may be referred to as a photo catalyst, for example. Materials that have been found suitable for use in the generation of oxygen from water include WO3 (see Erbs et al.), which may be combined with other materials for improved photocatalytic activity.
In many natural and artificial photochemical Z-schemes (a two-step photoexcitation system) a redox mediator is employed to facilitate the electrochemical generation of hydrogen and oxygen, allowing the large voltage gap between OER onset and HER onset to be traversed in two smaller steps. The use of a mediator removes the requirement that a single photo catalyst must simultaneously effect water oxidation and proton reduction, and instead couples separate, optimized photo catalyst ensembles for OER and HER together.
An example of a hydrogen and oxygen generating system in a photochemical Z-scheme is described by Maeda et al. Here, a IO3−/I− pair is used as a mediator between the oxygen evolution step, which utilities a Pt-loaded WO3 photo catalyst, and the hydrogen evolution step, which utilises a Pt-modified oxynitride photo catalyst. The entire process is performed in one pot, and the hydrogen and oxygen products are collected together.
The generation of oxygen using a modified WO3 catalyst is described by Miseki et al. Here, Fe3+ is used as the mediator, which is reduced to Fe2+ with concomitant production of oxygen. The authors focus on improving the generation of oxygen only. The reduced iron species, it is noted, may be regenerated as Fe3+ in a separate electrochemical step, using a photo catalyst and a sacrificial electron donor, such as an organic compound. Thus, the mediator is not linked to the production of hydrogen.
Both sacrificial (the mediator is destroyed as water is split) and recyclable mediator systems have been described. However, the majority of the mediators described (especially the most commonly used Fe2+/Fe3+ system) are incompatible with the polyelectrolyte membranes that would be required by practical electrochemical systems. Furthermore, the mediators are used in solution together with the photocatalysts, where they attenuate the light reaching the photoactive material and participate in unproductive back-reactions. Thus, typical mediator concentrations are in the low millimolar range, where OER and HER remain tightly coupled (one can only occur if the other also proceeds at an appreciable rate). This means that hydrogen and oxygen are necessarily produced simultaneously, with the rate of one half reaction dependent on the rate of the other usually OER is the rate determining step (RDS), being up to 1,000 times slower than HER.
There is a need for an electrochemical system and electrochemical methods for generating hydrogen and oxygen at low potentials. A system and a method that are suitable for use with a sustainably derived energy source are particularly desirable. A system that is adaptable for use in a standard electrochemical cell and a photo electrochemical cell would be particularly advantageous.